What is social bookmarking? It is the practice of saving bookmarks to a public website and "tagging" them with keywords. Tags are one word descriptors you assign to a bookmark to help organize the sites you collect, or you can think of them as keywords. These sites are accessible from any computer connected to the internet anywhere. Here is an article talking about what it is who is using it how it works, take a few minutes to read to gain a better understanding.
Educause 7 things you hould know about Social Bookmarking

Sites
del.icio.us
Diigo
Furl
Backflip